Instead of a `latestOnly` parameter, can we take an `options` object? That will have two benefits:

1. The code will be more readable. We'll get something like `fn(items, { latestOnly: true })`, which is easier to parse than `fn(items, true)`.

2. It'll help future-proof if we end up adding additional options. 

Alternately, we can keep it this way. If that happens, when `responsesToGeoJSON` is called with a `latestOnly` parameter, it would be helpful to pass it a constant for readability. Something like `LATEST_RESPONSES_ONLY` or `ALL_RESPONSES`, which would be assingned to `true` and `false`.